Supercharger mod. Riva Gen2 vs ET 127mm

Just trying to get peoples opinions on which wheel to run on my 215 RXP.
A guy local to me will supply a Riva Gen 2 wheel but i want peoples opinions on which is better. Riva gen 2 127mm vs ET 127mm
Why is it that the Riva 127mm does not require a 3mm spacer and the ET wheel does?
Which is better?
I need Help......

ET. There's a few boys down there that are running the 127. They've asked me many questions on setup and I'm sure you can ask them. I run a 140 now and at times wish I kept things simple with that wheel when I ran it but I'm too far gone now lol

jerry sells both in his store for whatever way you choose to go. I just share knowledge and results I had .. At the time I had a Riva s3 charger and was told to try this new wheel 4 years ago. So I did and wow .. Like I said just my experience here but the 127 hammered the s3 in mid range pull.
The 3 mm spacer is what some of the mid range differences. The older Riva wheels had a taller blade and a outer charger housing to compensate for that larger blade therefore a spacer was not needed but correct me if I'm wrong

i had looked at the Akit and the Akit-R. I heard Et127 had best performance at half the price. I would like to know why you feel the Akit is best as it has the lowest boost of the 3. Interested in your comparison of the 3.